ExtJS Using a Combobox in a popup window - forms

Question

I'm trying to add a combobox to a floating (popup) form panel in ExtJS. But I'm getting a "Cannot set property 'component' of null" error and the window will not load.
I use the following code in a controller to create the window:
onTreeAddDocClick: function () {
var f = new Ext.form.Panel({
frame: false,
header: false,
floating: true,
closable: true,
items: [{
xtype: 'addDoc'
}]
});
f.show();
}
The code for the window itself is as follows:
Ext.define('OPENhrm.view.dossier.widget.popup.AddDoc', {
extend: 'Ext.form.Panel',
xtype: 'addDoc',
requires: [
'Ext.layout.container.VBox'
],
controller: "dossier-addDoc",
viewModel: {
type: "dossier-addDoc"
},
id: 'addDocForm',
frame: true,
title: 'Add document',
width: 400,
bodyPadding: '10 10 0',
layout: 'form',
closable: true,
defaults: {
anchor: '100%',
allowBlank: false,
msgTarget: 'side',
labelWidth: 50
},
items: [{
// item selector
xtype: 'filefield',
emptyText: 'Select Document',
fieldLabel: 'Document',
name: 'filePath',
id: 'filePath',
buttonText: 'upload document',
buttonConfig: {
icon : '/resources/images/icons/add.jpg'
}
}, {
xtype: 'combo',
fieldLabel: 'Test',
hiddenName: 'test',
store: new Ext.data.SimpleStore({
data: [
['Test1'],
['Test2']
],
id: 0,
fields: ['text']
}),
valueField: 'text',
displayField: 'text',
triggerAction: 'all',
editable: false
}],
// buttons
buttons: [{
text: 'Add',
handler: 'onAddDockClick'
}, {
text: 'Reset',
handler: function () {
this.up('form').getForm().reset();
}
}]
});
If I remove the combobox, the window works just fine. If I place the combobox in a form somewhere else in my application (e.g. on a page with 2 panels; a searchfilter/form and a grid with search results), it works just fine. That other page however, is not a floating/popup window.
I got it to work by defining the whole page in the controller, but as I'm using a MVC structure, that doesn't seem like the way to go. Does anyone know how to get the combobox to work in a floating window, without putting the whole code for that window in the controller?